The present invention relates to printed circuit boards and more specifically to a multi-layer printed circuit board that contains an additional rework layer that simplifies the rework process thereof and increases the density of possible rework.
As the amount and complexity of integrated circuitry on a printed circuit board increases, so does the complexity of the signal layers and the number of layers required for the multi-layer printed circuit boards. The increased complexity, however, limits the amount of rework that can be accomplished using present rework techniques.
Also, as the density and complexity of integrated circuits increase, propagation time and the associated length and impedance of the signal paths becomes more critical. It is therefore necessary that rework wiring match and accomodate the differences in the impedance in a multi-layer board.
Presently, when problems arise that require re-wiring of a multi-layer printed circuit board, twisted pair wire is employed, with one conductor being grounded and the other used to make the repair; but the use of such wire requires heavier wire and twice the number of wire terminations, and the availability of a large number of ground terminations over that which is required by using the present invention. Thus, the density of possible rework is limited.
Examples can be found in the prior art demonstrating the use of separate wires either embedded in the circuit board or sewn through vias therein to make connections to components mounted thereon, reference U.S. Pat. Nos. 3,711,626 and 3,701,838. Examples can also be found of wire scribed circuit boards that serve as alternatives to multi-layer printed circuit boards, reference U.S. Pat. No. 3,674,602 and the Multiwire.RTM. reference guide (No. 05785000KC) of the Multiwire Division, Kollmorgen Corporation. Yet another example teaches the use of conductive patterns to equalize the impedances of signal layers spaced unequal distances from a conductive ground plane, reference U.S. Pat. No. 4,047,132.
The present invention borrows from this increasingly more complex technology and teaches an improved multi-layer printed circuit board that incorporates a dedicated rework layer which is generally comprised of a rework reference layer (i.e., at ground or some other potential) and a network of overlying wires semi-permanently affixed to the rework reference layer. The new combination thus enables the rework of the printed circuit board at any time and increases the density of possible rework by at least a factor of "two" over the previous cumbersome rework methods.